Currently approved spherical and cylindrical probes for photodynamic therapy have gained acceptance and wide use because they can uniformly irradiate internal body structures, deliver all of their optical power to the tissue of those structures and are capable of accurate calibration of the delivered dose. They accomplish this only when introduced into similarily shaped structures but are less successful when applied to single-sided surfaces such as a chest wall or the skin or to irregularly shaped structures. This proposal describes a new probe design which is expected to maintain the desirable features of the presently approved probes during these unsuitable conditions. The new probe takes the form of an inflatable bladder placed in contact or close proximity to the tissue to be treated. The first order design and performance of several variations of the basic configuration are analyzed. Optimization via optical design programs is outlined. Proposed fabrication and testing of the probe are detailed. The new probe will be adapted to the promising new treatment of malignant mesothelioma. PROPOSED COMMERCIAL APPLICATION: The applications of this probe lie in treatment of large area malignacies such as the malignant mesothelioma procedure and basal cell carcinoma and treatment of residual diseased areas after removal of tumors. There are 25 centers in the United States that would need one or more probes for mesothelioma. The number of basal cell procedures will be much greater.